[SB Refactor] Split out mobile's default information into two flows.

`defaultMobileConnectivity.isConnected` sounds like it means "is the
default mobile subscription connected?", when it actually means "is the
default connection a mobile one?". This CL splits out the
`defaultMobileNetworkConnectivity` flow into two flows with more
descriptive names.

Bug: 272586234
Test: all tests in mobile directory
Change-Id: Ie8d492c5983ce32bf22a0bca6d682a8b5cccf520